Multibeam Ka-band satellites for Broad-Band Services (BBS) allow efficient use of orbit-spectrum and power resources, reducing the cost of the space segment and making the service competitive. One of the major challenges being faced by multibeam BBS networks is how to maximize Operator revenues while coping with a highly uneven traffic demand distribution over the coverage region.
The basic principle of multibeam satellite communication systems is to cover a specified Region of Interest (ROI) with a number of independent beams. Each independent beam exploits a portion of the available bandwidth. The isolation resulting from antenna directivity and the spatial separation between beams is exploited to re-use the same frequency band in separate beams. The same spectrum can be so re-used multiple times over the ROI. In practice, frequency re-use is limited by the achievable beam to beam isolation and interference rejection. The frequency re-use factor is defined as the number of times that the same frequency band is used. Frequency re-use allows increasing the total capacity of the satellite system without increasing the allocated bandwidth. Moreover, using polarization diversity—i.e. of two orthogonal polarization states—allows doubling frequency re-use without increasing interferences.
The traffic demand (load) experienced by a beam corresponds to the cumulative sum of the spatial traffic demand density—i.e. the traffic demand for unit surface of a small portion of the ROI—over its coverage area (footprint).
Standard coverage design of multibeam satellite communication systems assumes a uniform traffic demand on which a regular coverage lattice is overlaid (see FIG. 1). A hexagonal lattice of contiguous circular spot beams of equal size is known in prior-art to be the most efficient to guarantee a minimum variation of the satellite antenna gain over a ROI. The usefulness of the hexagonal layout can be understood observing that the worst-case (lowest) beam gain corresponds with the triple cross-over points between adjacent beams (i.e. with the hexagon vertices) since they lie at the greatest distance from the beam centre (corresponding to the beam peak gain). Simple geometrical reasoning shows that for a same centre-to-vertex distance a regular hexagon has a substantially larger area and requires fewer beams to cover the ROI than any other partition, i.e. based on square or triangular cells. FIG. 2 shows a frequency/polarization reuse scheme using four different “colours” (i.e. frequency/polarization pairs, e.g. two disjoint frequency bands and two orthogonal polarization states) applied to a hexagonal lattice of cells.
In multibeam systems with regular beams layouts, however, a uniform spatial traffic demand density can be expected to be a rather unrealistic scenario and a real non-uniform spatial traffic demand density will create traffic load imbalances among different beams. These imbalances make some beams to be working at their maximum yet not meeting the demand (overloaded) while others use a fraction of their potential capacity (underloaded). This translates into a situation in which part of the system capacity is left unused while there is still demand to be met. The overall effect is a serious limitation to the usability of the total system capacity.
Recent developments allow dealing with this problem by allocating the available resources according to the beam traffic demand, i.e. by transferring the unused capacity from the underloaded beams to the overloaded ones (“Usable Capacity Transfer” concept). The leading idea is to have a common pool of radio-frequency (RF) resources from which a channel can adaptively draw the RF power it requires. The achievement of this objective is strictly dependent on the architecture of the high-power RF section.
A known technique for providing flexible RF resource allocation to beams depending on traffic demand consists in using Multiport Amplifiers (MPAs). The main characteristic of a multi-port amplifier is that the signals corresponding to each input port are first split by an input matrix in such a way that the signals of all the beams are amplified by all the high power amplifiers (HPAs) [1]. Therefore equal loading of all the HPAs is achieved independently of the beams power distribution. Depending on the input port (beam) considered, the input matrix generates different relative phases at the HPA inputs. Then the output matrix coherently recombines the signals belonging to each input port to the corresponding output port.
U.S. Pat. No. 6,091,934 discloses another technique to ensure dynamic allocation of power to satellite's high power amplifiers to maintain amplifier efficiency and meet peak traffic demands and reduce power consumption during low traffic periods. This can be accomplished adjusting the RF output power from a Travelling Wave Tube Amplifier (TWTA) by changing its bias conditions. Such TWTAs with variable saturated RF power are also known as Flex-TWTAs [2]. In payload configurations based on Flex-TWTAs each individual TWTA can arbitrarily change (within a range that in current developments is about 3-4 dB) its saturated RF output power to match the traffic demand while maintaining an almost constant efficiency. The Flex-TWTAs must be operated such that their aggregated power consumption does not exceed the overall available DC power.
Architectures based on Flexible Travelling Wave Tube Amplifiers (Flex-TWTAs) and/or Multi-Port-Amplifiers (MPAs) combined with RF switch matrices, which allow fixed bandwidth channels to be routed between amplifiers, can also provide a certain degree of flexibility ([3-5]).
In any case, these techniques result in added complexity which usually comes along with several impairments such as interference, intermodulation, power losses, spectral efficiency loss, etc.
Furthermore, flexibility in bandwidth allocation contrasts with the need to reduce number of TWTAs in multibeam payloads, problem that in current systems (e.g. Anik-F2, KaSat, etc.) led to the introduction of high-power demultiplexers (HP-DEMUX) to share a single TWTA among multiple beams. This problem is of particular importance for payloads requiring significant increase in the number of beams beyond the technology employed by current systems.
U.S. Pat. No. 6,173,178 and U.S. Pat. No. 6,813,492 disclose a method of generating a non-uniform coverage of a Region of Interest. According to this method, beams of at least two different sizes are used to cover to ROI, larger beams being directed to regions having a lower traffic demand density, and smaller beams being directed to regions having a higher traffic demand density. These documents, however, do not provide any systematic method of designing the coverage, and therefore do not allow optimizing the performances of the communication system.
The invention aims at solving or attenuating the above-mentioned drawbacks of the priori art.